Cardiac catheterisation is a minimally invasive clinical procedure in which tubes or thin wires are inserted into the patient's cardiovascular system via a peripheral blood vessel in order to treat and diagnose a wide range of conditions. Standard practice employs x-ray fluoroscopy to guide placement of the devices. However, this results in a large radiation dose and hindered device navigation as x-ray images have poor tissue contrast and are two-dimensional in nature.
MRI guidance of interventional procedures (iMRI) is an appealing alternative due to improved tissue visualisation and zero deposition of ionising radiation. However, two primary challenges remain.
Firstly, the radiofrequency electromagnetic fields used in MRI can induce currents on any conductive devices, such as guidewires and braided catheters. This can result in high focal heating, posing a risk to patients. Many solutions have been proposed. Fibreglass guidewires eliminate the risk, but suffer from reduced mechanical performance and can shatter. Another approach alters the architecture of interventional devices, changing their electromagnetic behaviour in order to ensure the radiofrequency fields cannot be absorbed. This approach also results in compromised mechanical properties. A final approach is to harness parallel transmission, an alternative MRI system architecture which enables enhanced control of radiofrequency fields. It is possible to create fields with the express purpose of minimising induced currents, informed either by electromagnetic simulations or monitoring currents on the device outside of the patient.
The second hurdle to be overcome is device localisation in order to enable navigation to the correct location. Several solutions have been proposed. Passive and active markers have been placed on devices to impart distinctive trackable ‘footprints’ in the MRI image. However this approach is not always robust due to variations in the background MRI signals. Another approach is to alter the image acquisition to sensitise the image to perturbations in the magnetic field caused by the device. However this strategy is dependent on the device having specific orientations in relation to the MRI magnet. Another strategy adds circuitry to devices to either embed or make the entire device act as an MRI receive antenna. This again can result in heating and results in non-standard guidewires. The final employed strategy embeds miniature resonant coils on devices, designed to perturb the radiofrequency field in order to create hyper-intense fields in their vicinity. This results in device visibility due to signal enhancement. This method has been further refined by tailoring the RF fields of multi-channel coils (such as those used for parallel transmission) in order to either maximise its measurement sensitivity to the marker signal, or to minimise the transmit field in the vicinity of the wire, so the images are only sensitive to signals produced by the marker.